Check if `text` is set to avoid IndexError (#4209)

Fix for https://github.com/huggingface/transformers/issues/3809

src/transformers/tokenization_roberta.py

@@ -236,7 +236,7 @@ class RobertaTokenizer(GPT2Tokenizer):
             add_prefix_space = kwargs["add_prefix_space"]
         else:
             add_prefix_space = add_special_tokens
-        if add_prefix_space and not text[0].isspace():
+        if add_prefix_space and len(text) > 0 and not text[0].isspace():
             text = " " + text
         return text